@Ondřej-Šutera sorry I missed your reply. I have updated my notifications settings to email me so I shouldn't happen again.
I could provided another screenshot but I'm not sure it will help you replicate it. I have been using Windy several times per week for the last few years and this bug has always existed. It happens on every model for every layer for every part of the map. It happens on my phone (Pixel 3a) but also the one other Android phone I have tested it on so far (Oppo Find X5). My timezone is Europe/London in case that matters.
Is it possible that the time slider is doing some calculation based on screen width that is being unreliably calculated? (I know getting window dimenions in a browser/WebView can be error prone.)
What is definitely true is that the logic to determine what time to display on the time slider is not the same logic to determine what time of data to fetch and display on the map. Even if you can't reproduce, I would guess that you would be able to verify this in code.
How can I best help you investigate this further - provide more screenshots or a video, or get a list of more devices that are affected, or something else?
Version is currently 39.3.2.